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Discuss Combi or system? Gas pipe issue. in the Gas Engineers Forum area at PlumbersForums.net

Hi All. I have an issue with a boiler in a small flat I purchased that has very old vented system boiler currently fitted.

Both boiler and water tanks are in a need of replacement.

I got 3 quotes from gas safe engineers and all recommended combi boiler to be fitted.

One of the engineers pointed out that the 15mm gas pipe supplying the current boiler is not up to standard with the combi requirement of 22mm.

But replacing the pipe is going to be a mess. It is laid under concrete floor and the meter is in another room. Will have to lay new pipe below the ceiling instead. It’s not going to be visually pleasing and I don’t want to do it. I’m considering going for new system boiler instead.

Engineer suggested that there is a chance new boiler will receive enough gas pressure through existing pipe, to function and meet minimum requirements as this is a small flat without other gas appliances. But there is no way to tell until the boiler is fitted.

If that proved to be the case we would leave 15mm pipe as is and install combi boiler. Otherwise we would have to replace the pipe first.

My question is if that is correct? Is there no way to measure gas pressure without purchasing the boiler?

If the gas pressure was too low and new pipe needed installing I would rather opt for unvented system boiler. Which would be probably much simpler to install and avoid gas pipe going through the living room.

So I wonder if there is a way to tell before purchasing anything, if gas pressure could be estimated/calculated without committing to any work.

And the other question do you think system boiler would be a reasonable option?

Any other advice welcomed!

1. To know whether 15 mm pipe would be adequate you need to know the distance (as the pipe is laid, not as the crow flies) between meter and appliance, together with the number of bends, tees, elbows between meter and appliance. Each fitting is roughly equivalent to 1/2 metre added to the length. Very roughly a 15 mm pipe will support a gas boiler of 24 kW provided it is no more than about 2.5 metres from the meter, including fittings. Pretty unlikely.
2. How has the size of the existing pipe been measured? It might be 15 mm at the boiler, but, given the pipework is under concrete it may well be steel pipe. Quite often this was laid in 3/4" pipe which has an internal diameter of around 19 mm. This would allow around 6 metres from the meter for the same rating of boiler, provided the 3/4" pipe was connected by 22 mm pipe for the final stretch to the boiler.
3. There is no realistic way to measure the pressure drop between meter and appliance without installing the appliance or a similar gas using device.
4. The gas usage, and hence the distance which can be served by 15 mm pipe, depends on the kW rating of the boiler (actually the cubic metres per hour of gas used, but they are equivalent). So you might find that even a system boiler would need new pipework, unless it were very small. Having said that, combi boilers are often rated for hot water supply, which requires a lot of gas for a short period. System (and open vent) boilers are generally sized for the heating requirement as the hot water is stored and can be heated gradually.

Timmy D

Gas Engineer
Does the current boiler still work? If so, an engineer can run it and test the pressure at the meter/boiler. This will give you a ball park.

What size is the pipe off of the meter?

A competent engineer should be able to pretty accurately pipe/boiler size with an on site visit.

Either way, without a new gas pipe, a combi won’t go in.

You’ll be fine with a small 12kw system boiler, if the gas supply copes.


Is there no (legal) way a gas installer can run off x lpm gas (equal to the maximum gas rate of the proposed new system boiler) and test the pressure at that flow, assuming the existing boiler is lower output than the proposed and therefore cannot be used as a reference?

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